Various methods and compositions for reducing the NO.sub.x concentration in the effluent from the combustion of a carbonaceous fuel have been proposed. For instance, Lyon, in U.S. Pat. No. 3,900,554, discloses the introduction of ammonia, as a pure substance or in the form of ammonium formate, ammonium oxalate or ammonium carbonate, into effluents at temperatures between 1300.degree. F. and 2000.degree. F. For operation at temperatures below 1600.degree. F., the use of reducing materials such as paraffinic, olefinic, aromatic and oxygenated hydrocarbons, as well as carbon monoxide and hydrogen, are required. Arand et al., in U.S. Pat. No. 4,208,386, disclose a method for reducing NO.sub.x in the combustion effluents by introducing urea, either as a solid powder or in solution, at effluent temperatures in excess of 1300.degree. F., also like Lyon with ancillary reducing materials at the lower temperatures. The preferred urea solutions according to Arand et al. are those having at least 10 weight percent urea.
Bowers, in U.S. Pat. No. 4,751,065, discloses the use of a solution which comprises urea and hexamethylenetetramine (HMTA) to reduce the NO.sub.x concentration in oxygen-rich effluents having temperatures above 1300.degree. F. Similarly, Bowers, in U.S. Pat. No. 4,719,092, discloses a solution comprising urea and an oxygenated hydrocarbon solvent which is disclosed as being effective at reducing NO.sub.x concentrations with reduced ammonia slippage in effluents at temperatures above 1600.degree. F.
Although the prior art processes for reducing nitrogen oxides concentrations are generally effective, there exists a present need for a process which elicits still further NO.sub.x reductions in an economical and convenient manner, especially over a broad range of effluent temperatures.